In an effort to meet stringent federal government emissions standards, engine systems may be configured with a plurality of technologies for decreasing emissions. Specifically, it may be desired to address nitrogen oxide (NOx) emissions. Two example technologies for decreasing NOx may include exhaust gas recirculation (EGR) and a selective catalytic reduction (SCR) device. Reducing NOx via EGR includes recirculating a controllable proportion of the engine's exhaust back into an intake passage to combine with intake air. The addition of EGR may not chemically participate in combustion (e.g., the gas is substantially inert) and may reduce an amount of cylinder contents available for combustion. This may lead to a correspondingly lower peak cylinder temperature and heat release. By doing this, NOx emissions may be decreased. Decreasing NOx via the SCR device includes a reductive reaction between NOx and ammonia (NH3) facilitated by the SCR device, which converts NOx into nitrogen (N2) and water (H2O). NH3 is introduced into an engine exhaust system upstream of an SCR catalyst by injecting urea into an exhaust pathway, or is generated in an upstream catalyst. The urea entropically decomposes to NH3 under high temperature conditions.
However, as recognized by the inventors herein, issues may arise upon flowing EGR into the intake passage and/or upon injecting urea into the exhaust pathway. In one example, EGR is introduced into the intake passage before the intake passage divides upstream of a multi-cylinder engine. Desired EGR mixing with intake air may be difficult to achieve at various engine speeds/loads, which may lead to uneven distribution of the EGR/intake air mixture. For example, one cylinder may receive too much EGR, possibly resulting in increased particulate emissions, and another cylinder may receive too little EGR, possibly resulting in increased NOx emissions. As a second example, urea may poorly mix with exhaust gas (e.g., a first region of exhaust gas has a higher concentration of urea than a second region of exhaust gas of an exhaust passage) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits. Thus, both introduction of EGR into the intake passage and introduction of urea in the exhaust passage may succumb to poor mixing.
Attempts to address insufficient mixing include introducing a mixing device at a junction between an EGR outlet and an intake passage and/or to introduce a mixing device downstream of a urea injector and upstream of an SCR device such that dispersion of EGR or urea may be more homogenous. Further attempts include one or more of adjusting valve function to introduce a backpressure to an intake and/or exhaust passage and/or to include injectors with swirlers and/or other turbulence generating features.
However, the inventors herein have recognized potential issues with such systems. As one example, a mixing potential of the devices may be limited. As an example, static mixers may be limited due to a velocity of gas flow due to their lack of vacuum or other mixing assisting features. Thus, the size of the mixers may be increased to overcome a dependence on gas velocity. However, increasing the size of the mixers may result in increasing a size of a gas passage, ultimately demanding significant modifications to a design of the gas passage. This may increase a production cost of a vehicle.
In one example, the issues described above may be addressed by a mixer comprising an outer annular portion exterior to an exhaust pipe and an inner annular portion interior to the exhaust pipe, where the outer annular portion comprises a spiral fin extending around the exhaust pipe in a downstream direction. In this way, exhaust gas may interact with inner and outer portions of the mixer to increase exhaust gas homogeneity.
As one example, the outer annular portion may comprise an outer passage located between the exhaust pipe and an outer surface. The spiral fin may adjust exhaust gas flow through the outer passage before directing exhaust gas flow to the inner annular portion in the exhaust passage. The inner annular portion may constrict the exhaust passage, wherein a vacuum is generated along annular perforations of the inner annular portion. This vacuum may promote exhaust gas flow into the outer and inner annular portions. By doing this, exhaust gas mixing may increase as exhaust gas flows through the various portions of the mixer. This may also occur in conjunction with a reductant injection. As such, reductant mixing into the exhaust gas upstream of an exhaust device may increase.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.